The Senate Judiciary Committee completed its marathon hearings on the Supreme Court nomination of Judge Robert H. Bork yesterday as President Reagan stepped up his personal effort to salvage the nomination.

With the committee scheduled to vote on the issue Tuesday, Reagan told a group of supporters in the Old Executive Office Building that Bork is neither a conservative nor a liberal but is "America's nominee to the Supreme Court."

The president said Bork "would not advance my political views" if confirmed to the high court. He emphasized law and order issues, a theme White House strategists hope will rally conservatives behind the beleaguered nomination.

Law enforcement officers support Bork, Reagan said, because "criminals terrorize the streets in too many of America's cities" and Bork was interested in public safety "and not just the protection of the criminal."

As the Bork hearings limped to an end after 12 days of testimony from 110 witnesses, Judiciary Committee Chairman Joseph R. Biden Jr. (D-Del.) released a strongly worded letter from former president Jimmy Carter, who called Bork's views on civil rights issues "obnoxious" and urged rejection of his nomination.

"As a southerner who has observed personally the long and difficult years of the struggle for civil rights for black and other minority peoples, I find Judge Bork's impressively consistent opinions to be particularly obnoxious," Carter said.

Carter said he wrote the letter in response to testimony supporting Bork's confirmation by "some prominent lawyers who served in my administration." He was referring to his fellow Georgian, former attorney general Griffin B. Bell, and former White House counsel Lloyd N. Cutler.

Meanwhile, the Bork confirmation fight increasingly took on the look of a full-fledged political campaign with each side in search of that elusive but critical quality known as momentum.

Two leading liberals, Majority Whip Alan Cranston (D-Calif.) and Sen. Patrick J. Leahy (D-Vt.), a Judiciary Committee member, delivered long speeches on the Senate floor announcing they would vote against Bork. Sen. John Glenn (D-Ohio) also announced he would vote against the nomination. Republicans countered with a statement from Sen. Richard G. Lugar (R-Ind.), a leading GOP senator, announcing that he would support confirmation.

With the hearings complete there is likely to be a stampede of senators announcing how they will vote in the final showdown on the Senate floor. Sen. Terry Sanford (D-N.C.), one of four freshmen southern Democrats who owes his election to overwhelming support by black voters is expected to announce his opposition today and Sen. David H. Pryor (D-Ark.) is expected to announce how he will vote.

Amid the maneuvering, Sen. Bob Packwood (R-Ore.), the only announced GOP opponent to the nomination, said at least three and as many as five Republicans will vote against confirmation. But Senate Minority Whip Alan K. Simpson (R-Wyo.) appeared in the Senate press gallery to deny rumors that he had privately described the Bork nomination as "doomed."

"It's the goofiest thing I've ever heard," he said. "I think Robert Bork is going to be confirmed."

Simpson also disputed the estimate Tuesday by Cranston, his Democratic counterpart, that 49 senators are now likely to oppose Bork's confirmation, 40 likely to support it and that 11 senators are undecided.

Simpson said his vote count showed Bork holding a four-vote lead with 20 to 24 senators undecided.

Cranston's estimate was also sharply disputed by White House chief of staff Howard H. Baker Jr., a former Senate majority leader. Warming up Reagan's audience, Baker said he had "genuine affection" for Cranston but called him "the worst vote counter in the U.S. Senate today."

Cranston retorted later that Baker "has lost touch with the Senate."

In his remarks, Reagan said the Bork confirmation fight centered on three choices:

"The choice between liberal judges who make up the law or sound judges who interpret the law. The choice between liberal judges whose decisions protect criminals or firm judges whose decisions protect the victims. The choice between liberal judges selected by the liberal special interests or distinguished judges selected to serve the people."

Reagan's remarks reflected a White House strategy of emphasizing criminal justice issues in the confirmation campaign rather than dwelling on other "social issues" such as abortion, on which Bork's views are less popular.

"We're trying do now what our opponents have been doing all along, which is to emphasize the strong points of our case," said one strategist.

White House officials, who conceded early in the week that they had taken a pounding in the national political campaign waged by Bork's opponents, were more optimistic yesterday. One official said that several southern Democrats who are being counted as anti-Bork votes "could turn out to be surprises."

The official said "we take Sen. {Robert C.} Byrd at his word when he says he hasn't made up his mind how he will vote." Byrd, the West Virginia Democrat and Senate majority leader, has urged that the Senate Judiciary Committee send Bork's name to the full Senate without a recommendation.

At the daily White House briefing, where many of the questions dealt with Bork, spokesman Marlin Fitzwater carefully refrained from criticizing Byrd's approach. Fitzwater said the White House view was that the full Senate should vote Bork "up or down."

Bork yesterday continued to meet privately with senators, including Arlen Specter (R-Pa.), the only uncommitted GOP member of the Judiciary Committee. White House officials said Reagan had not yet called any undecided senators but would be heavily involved "when it gets down to crunch time in the full Senate."

Yesterday's testimony offered a spectrum of witnesses. An Orthodox rabbi joined Beverly LaHaye of Concerned Women for America and Roy Innis of the Congress of Racial Equality in support of Bork, while small-business representatives, Chicago lawyers and black law enforcement groups were among those opposing him.

A panel of antitrust experts, countering the pro-Bork antitrust panel Tuesday, testified that Bork's views on antitrust were far outside the mainstream and reflected a willingness to engage in judicial activism and disregard congressional intent.Staff writer Ruth Marcus contributed to this report.